The present invention relates to an apparatus for measuring the transfer characteristic of a servo system or filter, or the impedance of a capacitor or coil.
Conventionally, such a measuring apparatus as mentioned above has an arrangement in which a sine-wave test signal is applied to a measuring object to derive therefrom two signals related to an item of measurement thereof, the two related signals are multiplied by a sine-wave (or square-wave) signal in-phase with the test signal and a sine-wave (or square-wave) signal leading the test signal by a phase angle of 90.degree. in analog form, respectively, the respective multiplied outputs are integrated for the period of an integer cycle from the time point of a zero phase of the test sine-wave signal after the generation of a measurement start signal, the two integrated outputs, i.e. two vector detected signals are A-D converted, and both output data are used to calculate the desired item of measurement.
FIG. 1 illustrates an example of such a conventional transfer characteristic measuring apparatus, in which a signal generating part 210 generates a sine-wave test signal TS, a sine-wave signal SI (or square-wave signal SQ) in-phase with the test signal TS, a sine-wave signal CO (or square-wave signal CQ) leading the test signal TS by a phase angle of 90.degree., and a synchronizing signal SS synchronized with the test signal TS, as shown in FIG. 2, and the test signal TS, is applied to a device under test 100. The sine-wave signal CO will also be referred to as cosine-wave signal. The sine-wave test signal TS, which is an input voltage of the device under test 100, is taken out as the one related signal A via a buffer 221, whereas the output voltage of the device under test 100 is taken out as the other related signal B via a buffer 223. The related signal A and the sine-wave signal SI (or square-wave signal SQ), the related signal A and the cosine-wave signal CO (or square-wave signal CQ), the related signal B and the sine-wave signal SI (or square-wave signal SQ), and the related signal B and the cosine-wave signal CO (or square-wave signal CQ) are multiplied in analog multipliers 231, 232, 233 and 234, respectively.
The synchronizing signal SS and a measurement start signal ST which is produced by a measurement starting operation are provided to a control part 240. The control part 240 applies an integration control signal IC to integration control switches S1 through S4 provided at the input terminals of integration circuits 251 through 254. As shown in FIG. 2, the integration control signal IC is made high for a period Pc of an integral multiple of a complete cycle of the test signal TS starting with zero after the generation of the measurement start signal ST. During the period Pc the integration control switches S1 through S4 are held ON, through which the output signals of the analog multipliers 231 through 234 are fed to the integration circuits 251 through 254 for integration.
Thereafter, a select switch 260 is switched from one fixed contact point to another under control of a switching signal SW from the control part 240 and the output signals of the integration circuits 251 through 254 are sequentially converted by an A-D converter (or "ADC") 270 to digital form accordingly. The output data of the A-D converter 270 are subject to calculation in a calculation part (or "COMP") 280 to obtain the results of measurement of the device under test (or "DUT") 100, which are displayed on a display part (or "DISP") 290. After the A-D conversion of the output signals of the integration circuits 251 through 254 in the A-D conversion part 270 reset switches R1 through R4 provided in the integration circuits 251 through 254 are momentarily turned ON by a reset signal RS from the control part 240, by which the integration circuits 251 through 254 are initialized.
Letting input and output voltages of the device under test 100 be represented by EQU Vin=Rin+jIin (1) EQU and EQU Vou=Rou+jIou (2)
the transfer characteristic of the device under test 100 is expressed as follows: ##EQU1## where Rin and Rou are real parts and Iin and Iou are imaginary parts.
In the above conventional measuring apparatus the related signal A corresponding to the input voltage of the device under test 100 and the sine-wave signal SI (or square-wave signal SQ) are multiplied in analog form and the multiplied output is integrated for the period Pc, by which the real part Rin in Eq. (3) is vector-detected. The related signal A and the cosine-wave signal CO (or square-wave signal CQ) are analog multiplied and the multiplied output is integrated for the period Pc, by which the imaginary part in Eq. (3) is vector-detected. The related signal B corresponding to the output voltage of the device under test 100 and the sine-wave signal SI (or square-wave signal SQ) are analog multiplied and the multiplied output is integrated for the period Pc, by which the real part Rou in Eq. (3) is vector-detected. The related signal B and the cosine-wave signal CO (or square-wave signal CQ) are analog multiplied and the multiplied output is integrated for the period Pc, by which the imaginary part in Eq. (3) is vector-detected. The transfer characteristic of the device under test 100 is obtained through calculation of Eq. (3) in the calculation part 280 with the use of the vector-detected outputs.
More specifically, in the case where the sine-wave signals SI and CO are employed, if the angular frequency of the sine-wave test signal TS is taken as .omega., the sine-wave signals SI and CO are represented by sin.omega.t and cos.omega.t, respectively, and if the amplitude and the phase of the related signal A are taken as a and .phi., respectively, the signal A is represented by a.multidot.sin(.omega.t+.phi.). Assuming that the period Pc is an n cycle or cycles (where n =1, 2, . . .) of the test signal TS starting from the time point of its zero phase, the vector-detected outputs are as follows: ##EQU2## Letting the amplitude and the phase of the related signal B be represented by b and .theta., respectively, the signal B is expressed by b.multidot.sin(.omega.t+.theta.). Hence, the vector-detected outputs are similarly given as follows: ##EQU3##
In the case of measuring the impedance of the device under test 100, the difference between the input and output voltages of the device 100, that is, the voltage across the device 100, is extracted as the one related signal A and the current flowing through the device 100 is extracted as the other related signal B.
That is to say, letting the voltage across the device under test 100 and the current flowing therethrough be represented by EQU V=Rv+jIv (8) EQU I=Ri+jIi (9)
the impedance of the device under test 100 is expressed as follows: ##EQU4## where Rv and Ri are real parts and Iv and Ii are imaginary parts. Accordingly, the related signal A corresponding to the voltage across the device under test 100 and the sine-wave signal SI (or square-wave signal SQ) are analog multiplied and the multiplied output is integrated for the period Pc, by which the real part Rv in Eq. (10) is vector-detected. The related signal A and the cosine-wave signal CO (or square-wave signal CQ) are analog multiplied and the multiplied output is integrated for the period Pc, by which the imaginary part IV is vector-detected. The related signal B corresponding to the current flowing through the device under test 100 and the sine-wave signal SI (or square-wave signal SQ) are analog multiplied and the multiplied output is integrated for the period Pc, by which the real part Ri in Eq. (10) is vector-detected. The related signal B and the cosine-wave signal CO (or square-wave signal CQ) are analog multiplied and the multiplied output is integrated for the period Pc, by which the imaginary part Ii is vector-detected. By performing the calculation of Eq. (10) in the calculation part 280 through use of these detected outputs, the impedance of the device under test 100 is obtained.
As described above, according to the conventional measuring apparatus, the analog-multiplied outputs of the two related signals A and B corresponding to the item of measurement of the device under test 100, the sine-wave signal SI (or square-wave signal SQ) in-phase with the sine-wave test signal TS and the sine-wave signal CO (or square-wave signal CQ) leading the test signal TS by a phase angle of 90.degree. are respectively integrated for the period Pc of an integer cycle of the test signal TS from the time point of the zero phase of the test signal TS after the generation of the measurement start signal ST to perform the vector-detection of the related signals A and B. Hence, the prior art apparatus is defective in that measurement consumes much time especially when the frequency of the test signal TS is low and that the accuracy of measurement is not satisfactory owing to limited accuracy of the analog multipliers 231 through 234 in the case of using the sine-wave signals SI and CO as detecting signals.
The inventor of this application has proposed in his U.S. Pat. No. 4,947,130 (filed Dec. 14,1988) an impedance measuring apparatus which performs a synchronous detection by use of a multiplying type D/A converter with a view to improving the accuracy of measurement. In the impedance measuring apparatus, sine-wave test data read out of a waveform memory is D-A converted and is applied to a measuring object to derive therefrom a related signal corresponding to its output current, and at the same time, digital sine and cosine waves are read out of different waveform memories and are each multiplied in the multiplying type D/A converter by the related analog signal from the measuring object. With this arrangement, the accuracy of measurement can be increased. However, since the integration period of each multiplied output is determined by counting a predetermined number of times an overflow of an accumulator for generating an address which is used to read each waveform memory, that is, since the integration period is selected to be a period equal to an integral multiple of the cycle of the test sine wave and starting at the zero phase thereof, the measuring instrument disclosed in the above-mentioned United States patent also has the shortcoming that the measurement is time-consuming.